1. Field of the Invention
This invention relates to laser resonators and gas laser systems in which a multiply folded optical path is extended in length with a simultaneous decrease in distributed optical losses, and more particularly, to the use of CO.sub.2 gain sections as power amplifiers, and the operation of CO.sub.2 lasers as mode-locked oscillators.
2. Background Art
Gas lasers have limited output power capability per unit length. The typical upper limit for a waveguide CO.sub.2 laser is of the order of 0.5 W/cm of gain length or, equivalently, 10.0 W/cm.sup.3 of excited gas volume. For practical output powers of the order of 50 W, a resonator length of the order of 1.0 m is required, which is a prudent practical upper limit for the physical opto-mechanical length of laser resonators that can be used while providing reliable, stable, and rugged opto-mechanical structures.
When lasers are required with higher output power capability, several techniques have been developed which allow an increase in either laser gain volume or resonator length.
U.S. Pat. No. 4,719,639 (Tulip) and U.S. Pat. No. 4,939,738 (Opower) show techniques which increase the output power of a CO.sub.2 laser by increasing the waveguide bore along one dimension to several times its original size, obtaining a slab discharge geometry. To increase the uni-dimensional waveguide size n-times, the output power increases to n-times the output power of the original waveguide laser. The large aspect ratio (ratio of major to minor bore dimensions) of this type of laser cavity requires the use of unstable resonator optics which, in general, produce an asymmetric output beam (in both beam size and divergence), which requires additional external optics, as shown in the Opower patent, to reformat the beam into a symmetric Gaussian profile, as required for practical use, in many applications.
U.S. Pat. Nos. 4,807,232; 4,807,233; 4,807,234; and 5,079,773 (Hart et al.) teach how to increase the output of a waveguide CO.sub.2 laser by using a multiplicity of parallel waveguide channels which are separated by incomplete side walls. Energy from each of the waveguide channels leaks into the neighboring channels at the wall gaps, and this interaction causes the output of the individual channels to be phase-locked with respect to each other. The leaky wall represents a loss for each individual waveguide, but this loss is required for the waveguide channel array to operate in a phase-locked mode. Successful operation of the waveguide array depends critically on the degree of coupling between adjacent waveguide channels, which is in turn rigidly determined by the degree of incompleteness of the side wall, and by the separation of the waveguide channels in the array. Increasingly higher output powers can then be achieved either by increasing the length of the waveguide array, or the number of waveguide channels in the array.
U.S. Pat. No. 4,813,052 (DeMaria) shows a similar technique in which the array is achieved by means of adjacent ridges, in either a planar or cylindrical geometry. Again, increased output power can be achieved either by increasing the length of the ridged waveguide, or the number of ridges in the array. For proper mode discrimination and successful phase-locked operation, the separation between adjacent ridges must exceed a minimum value. Since the entire gas volume is electrically excited, but only the region between ridges is optically active, the efficiency of the ridged waveguide (ratio of output optical power to input electrical power) is low, with respect to the efficiency of the equivalent single waveguide channel with complete side walls.
There are a number of applications of CO.sub.2 laser technology that require an increase in laser power that can be achieved solely by an increase in resonator length. For these applications, the aforementioned techniques based on transverse scaling or multiple bore arrays are not applicable. Two typical examples are the use of CO.sub.2 gain sections as power amplifiers, and the operation of CO.sub.2 lasers as mode-locked oscillators.
R. J. Carbone, in "Characteristics of a Single-Frequency Sealed-Off CO.sub.2 Amplifier", IEEE Journal of Quantum Electronics, Vol. QE-5, Pages 48-49, January 1969, uses a large bore, low pressure CO.sub.2 gain tube comprising three parallel 3-meter sections linearly coupled by suitable fold mirrors to amplify low power CO.sub.2 radiation. With 1.0 W of input power, the 9-meter structure produces a total of 95 W in sealed-off mode, and 110 W in a flowing gas mode of operation.
Similarly, M. B. Klein and R. L. Abrams, in "10.6-.mu.m Waveguide Laser Power Amplifiers", IEEE Journal of Quantum Electronics, Vol. QE-11, No. 8, August 1995, describe the use of a DC-excited waveguide structure with flowing gas, for the amplification of the output of low power CO.sub.2 radiation. Starting with an input power of 1.0 W, they determined that the amplifier lengths required to obtain the target output powers of 100 and 200W were 5.0 and 9.0 m, respectively. If the amplifier were required to operate in a sealed-off mode with no gas flow, the required amplifier length would increase considerably, due to the lower small signal gain coefficient in this mode of operation.
In mode-locked operation, periodic cavity losses are introduced in the laser cavity, usually by means of intra-cavity modulators. If the loss-producing waveform has a period which matches the photon round trip time of the resonator cavity, the output is mode-locked and comprises a train of narrow pulses separated by a time interval equal to the cavity round trip time. One of the most useful applications of this type of laser waveform is for Range-Doppler Imaging (RDI) laser radar (LADAR) systems. In these systems, targets can be resolved in range (i.e., size) and frequency (i.e., spectral characteristics due to rotation and precession) by analyzing, in a time-gated receiver, the radiation such targets reflect when illuminated by a laser beam. For unambiguous results, only one laser pulse at a time must illuminate the target. Extended targets several meters in length require a mode-locked waveform with long pulse periods, which can only be produced by laser resonators with long cavity lengths. Since the prevailing use of these mode-locked laser systems is for either tactical or strategic military applications, the opto-mechanical structure of long cavity length resonators must be further reduced in order to perform satisfactorily under severe environmental conditions and stressful deployment modes.
U.S. Pat. Nos. 4,429,398, and 4,438,514 (Chenausky et al.) show techniques to increase the length of waveguide CO.sub.2 lasers by either using two parallel waveguide channels which are U-folded, or three waveguide channels which are Z-folded by means of intra-cavity folding optics. For a given resonator length, these techniques produce a two or three-fold decrease in longitudinal dimensions with a moderate increase in lateral dimensions.
U.S. Pat. No. 4,815,094 (Cantoni) and U.S. Pat. No. 4,870,654 (Cantoni et al.) demonstrate that a large reduction of the resonators longitudinal dimension can be achieved by multiple intra-cavity folding techniques, wherein the intra-cavity laser paths intersect. Increasingly longer resonator lengths can then be achieved by increasing the separation between waveguide channels for a given number of intra-cavity optical folds, or by increasing the number of folds for a given separation between waveguide channels. Conversely, smaller structures can be achieved for a given resonator length, by decreasing the separation between waveguide channels while, at the same time, increasing the number of intra-cavity optical folds. The first technique results in larger opto-mechanical structures, which may not be acceptable in some applications. The second technique leads to an increase in intracavity losses, which results in a smaller increase in laser output power than would be predicted by the linear increase in cavity length. The complete text of both of these patents are hereby incorporated by reference into this specification.